1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as a copying machine, a printer, a facsimile machine, or a multifunction printer integrating such devices, and more particularly to an image bearing member drive unit for driving an image bearing member provided in an image forming apparatus, such as a photosensitive drum.
2. Description of the Related Art
In general, the electrophotographic process executed by an image forming apparatus includes an image formation process, and the image formation process includes an electrostatic charging process, an image forming process, a development process, a transfer process, a cleaning process, an electrostatic charge removal process, and the like.
In the electrostatic charging process, a surface of a photosensitive drum as an image bearing member is electrostatically charged, and in the image forming process, an electrostatic latent image is formed on the photosensitive drum according to image information obtained from an external device, such as a scanner or a personal computer. In the development process, the electrostatic latent image is developed as a toner image using a developing agent, such as toner.
Then, in the transfer process, the toner image on the photosensitive drum is transferred onto a transfer material. After the transfer process, the cleaning process is executed to remove remaining toner on the photosensitive drum. Thereafter, the electrostatic charges on the photosensitive drum are removed by the electrostatic charge removal process, and then the electrostatic charging process is executed again.
By the way, in black-and-white printing, a recording sheet (recording medium) is used as a transfer material for the transfer process, and the toner image on the photosensitive drum is directly transferred onto the recording sheet.
On the other hand, in an image forming apparatus capable of performing color printing (hereinafter referred to as a color image forming apparatus), the transfer process is executed to transfer toner images from photosensitive drums for respective colors onto an intermediate transfer belt as a transfer material, in a superimposed manner, to thereby form a color toner image on the intermediate transfer belt. Then, the color toner image is transferred from the intermediate transfer belt onto the recording sheet.
Although in the image formation process, the photosensitive drum and the intermediate transfer belt are driven for rotation at a constant surface speed (hereinafter referred to as a processing speed), color shift and banding may be caused on an image formed on the recording sheet.
The causes of color shift and banding include fluctuations in respective processing speeds of the photosensitive drum and the intermediate transfer belt, and variation in exposure position due to shaking of an exposure device during the image forming process.
Particularly, the fluctuations in the processing speeds of the photosensitive drum and the intermediate transfer belt are a major cause of color shift and banding in image forming, development, and transfer (primary transfer and secondary transfer) in the image formation process.
Therefore, to reduce color shift and banding, it is very important to control the processing speeds of the photosensitive drum and the intermediate transfer belt to constant speeds (hereinafter referred to as constant-speed control).
However, in the color image forming apparatus, various speed fluctuation factors exist in the entire drive system. This makes it difficult to ideally control the processing speed of the photosensitive drum to a constant speed.
For example, the speed fluctuation factors include an eccentricity component of a photosensitive drum shaft, a torque fluctuation component of a motor, eccentricity and vibration components caused by a reduction unit, such as a gear, and a random load fluctuation component. Here, an example of the random load fluctuation component includes fluctuations in frictional force between the photosensitive drum and the intermediate transfer belt.
On the other hand, a motor used as a drive source of the photosensitive drum or the intermediate transfer belt is generally implemented by a brushless DC motor. The brushless DC motor is employed because of the advantages that it can be easily rotated at high speed, and it is free from step-out occurring in a stepper motor.
In executing the constant-speed control using the brushless DC motor, a so-called PLL (phase-locked loop) method or a speed discriminator method is used. In the speed discriminator method, speed feedback control using an FG (frequency generator) signal (rotation pulse signal) is carried out. In the PLL method, speed and phase feedback control using an FG signal is carried out. Further, a drive circuit converts a controlled variable output by the constant-speed control to current for driving the motor.
Here, for example, as a circuit for driving the image bearing member, a so-called bidirectional drive circuit using transistors or FETs as switching elements is used.
In the bidirectional drive circuit, the magnitude of current applied to a motor coil is controlled by a pulse width modulation method (hereinafter referred to as the PWM method), and is determined by a ratio of an on duration in which a voltage applied to a gate or a base of a switching element is on to a total of the on duration and an off duration in which the same is off (hereinafter referred to as a duty factor). That is, the bidirectional drive circuit outputs a controlled variable determined by the feedback control, in the form of duty factor, so as to carry out the constant-speed control.
However, in current control by the PWM method, there is a difference in response (time required for the speed to reach a target speed) between acceleration (acceleration response) and deceleration (deceleration response) of the motor. In acceleration, it is possible to increase the accelerating force by increasing current applied to the motor.
On the other hand, in deceleration, the limit of the decelerating force is determined by a mechanical frictional force, and hence the deceleration response is degraded in the image bearing member having a large inertia.
For example, there has been proposed an image forming apparatus which applies a braking force to a rotating shaft using a mechanical friction brake so as to improve the deceleration response (see Japanese Patent Laid-Open Publication No. 2003-195687). In this image forming apparatus, load fluctuations of a photosensitive drum are measured by a rotational load measurement unit and are stored in a rewritable memory in advance. Then, brake control is carried out according to the load fluctuations so as to improve the deceleration response.
Further, for example, there has been proposed a technique which uses a short brake to improve the braking force (see Japanese Patent Laid-Open Publication No. H11-27979). In this technique, a braking force is generated in a motor itself using negative current (electric current which generates a torque in a direction opposite to the direction of rotation of the rotating shaft) which is generated by an induced electromotive force during deceleration.
This improves the deceleration response even when the degree of attenuation of current is small. Further, according to the technique disclosed in Japanese Patent Laid-Open Publication No. H11-27979, the constant-speed control is carried out such that the PWM period is separately set in a section of the rotation driving operation and a section of the short brake operation.
As described above, when the constant-speed control of the image bearing member is performed, the deceleration response is lower than the acceleration response, and as a result, it is difficult to properly control high-frequency speed fluctuations. To cope with speed fluctuations, load fluctuations are measured in advance, and a mechanical frictional force is applied to a rotating shaft of the image bearing member, whereby feedforward control is carried out.
However, when it is attempted to improve the deceleration response by using the mechanical frictional force, not only the costs but also energy loss is increased due to a mechanism provided for applying the frictional force. Further, the feedforward control makes it difficult to cope with random disturbances produced during the image formation process.